(1) FIeld of the Invention:
The present relates generally to lift systems for agricultural implements, and more specifically, to a hydraulic lift system raising and lowering multi-section implements between raised transport and lowered depthcontrolled working positions.
(2) Related Art:
Large tillage implements often have a lift and depth control circuit such as depicted in FIG. 1 of in U.S. Pat. No. 3,663,032, of common ownership with the present application. Two lift cylinders connected to the lift wheel assemblies on the main frame of the implement are plumbed in parallel with each other and are connected through a mechanical member such as a rockshaft. The rockshaft keeps the opposite sides of the implement raising and lowering in unison and keeps a substantially uniform depth across the implement. Outrigger or wing frames pivotally connected to the opposite ends of the main frames each include a lift assembly with a cylinder connected in series with the nearest rockshaft cylinder to maintain a fairly uniform height across the width of the implement. However, uneven vertical forces across the width of the implement often cause rockshaft torsional wind-up which results in uneven depth, damage to the rockshaft, and side forces making steering of the towing vehicle more difficult. The problem of uneven forces is compounded since the wind-up of the rockshaft and the resulting change in displacement of the rockshaft cylinders causes the corresponding wing cylinders to move in the same direction. If the wing on one side starts digging in too deeply, the rockshaft cylinder which is connected in series with the wing cylinder will cause the corresponding side of the main frame to dig in more deeply also while the opposite side of the main frame will actually decrease in depth. Therefore the system is unstable in that initial reaction of uneven load tends to cause the loading to become more uneven. Although circuits have been proposed which hydraulically isolate the left-hand and the right-hand circuits when operating in the field, these circuits have added cost and complexity to the hydraulic system.